2.9BSD/usr/lib/learn/eqn/L1.1d

#print
The "-ms" package lets you do some other nice things as
well.  For example, you can arrange for a number to be
placed on an equation, in the right margin:

  a = b + c - d                               (1.23)

by placing the number after the .EQ on the same line,
like this:

.EQ (1.23)

You must leave a space between .EQ and the number.

In the file "Example", number the equation "17.1a"
and print it properly with neqn and nroff -ms.
(Don't forget -Txxx for your terminal.)
Then type "ready".
#once #create Example
.PP
This is a tiny little text
that includes one equation
or at least it will when you find this line:
.EQ
x sub i = y sub i + z sub i
.EN
and convert it into an equation.
(Naturally I changed it again.)
.pl 1
#user
grep '\.EQ  *17\.1a$' <Example >/dev/null
#log
#next
1.1e 10